1. Field of the Invention
The present invention relates generally to handoff between heterogeneous networks. More particularly, the present invention relates to a handoff method for improving delay and packet loss which may frequently occur when a mobile node (MN) performs handoff from a Wireless Local Area Network (WLAN) network to a Universal Mobile Telecommunication System (UMTS) network.
2. Description of the Related Art
Mobile (Internet Protocol (IP)) Working Group of Internet Engineering Task Force (IETF), an international standardization organization, proposes a pre-registration handoff method as one of the methods capable of supporting a low delay during handoff. The pre-registration handoff method refers to a method in which a Mobile Node (MN) performs L3 (Network Layer) handoff before completion of L2 (Link Layer) handoff with the help of a network. As referenced to herein, the L2 handoff refers to an operation of switching a Foreign Agent (FA) that the MN accesses, and the L3 handoff refers to an operation of registering an IP address of the MN in a Home Agent (HA).
FIG. 1 depicts a signaling diagram illustrating a conventional pre-registration handoff process. Referring to FIG. 1, an MN 110 desires to perform pre-registration handoff from an old FA (oFA) 120 to a new FA (nFA) 130.
Before the pre-registration handoff is performed, the oFA 120 transmits an Agent Solicitation message for requesting information about the nFA 130, step 101. Upon receiving the Agent Solicitation message from the oFA 120, the nFA 130 transmits an Agent Advertisement message including information on the nFA 130 back to the oFA 120, step 102. The MN 110 acquires information on the nFA 130 through the Agent Advertisement message.
Thereafter, if the MN 110 or the oFA 120 receives an L2 trigger, step 107, handoff is triggered (initiated) at the MN 110 or the oFA 120. Specifically, if the MN 110 receives the L2 trigger in step 107, the handoff is triggered by the MN 110. Upon receiving the L2 trigger, the MN 110 transmits a Proxy Agent Solicitation message to the oFA 120 in step 103. Upon receiving the Proxy Agent Solicitation message, the oFA 120 transmits a Proxy Agent Advertisement message to the MN 110 in response thereto, step 104. If the oFA 120 receives the L2 trigger in step 107a, however, the handoff is triggered by the oFA 120. That is, upon receiving the L2 trigger, the oFA 120 transmits a Proxy Agent Advertisement message to the MN 110 in step 104.
Upon receiving the Proxy Agent Advertisement message, the MN 110 transmits a Registration Request message to the nFA 130 via the oFA 120 in step 105 because it is not yet connected to the nFA 130. Upon receiving the Registration Request message, the nFA 130 registers the MN 110 in an HA 140 through a Mobile IP registration process, in step 106, which is the L3 handoff. Thereafter, if the HA 140 delivers a Registration Reply message to the MN 110 via the nFA 130 and the oFA 120 in step 108, the L2 handoff is initiated in step 109. The L2 handoff allows the MN 110 to access the network via the nFA 130 instead of the oFA 120.
The foregoing pre-registration handoff scheme separates the L2 handoff and the L3 handoff, performing first the L3 handoff (that is, the Mobile IP registration process), which may cause a long handoff delay and thereby contribute to a reduction in the total handoff delay.
The pre-registration handoff scheme, which operates based on Mobile IP, is not flexible when applied to a WLAN-UMTS interworking configuration, and must be adapted to perform an actual handoff process in WLAN and UMTS networks, as well as meet the requirements of the networks. That is, during WLAN-UMTS interworking the Mobile IP registration process includes an authentication process, whereas the conventional handoff scheme presents only the mobility-related registration signaling but suggests nothing about the authentication process. This causes a considerable delay and packet loss problem during actual handoff between the WLAN and UMTS networks.
Accordingly, what is needed is a system and method capable of providing fast, seamless handoff during WLAN-UMTS interworking.